This invention relates generally to human hip replacement, and more particularly to achieving more accurate leg positioning and joint stability, during surgery. The invention concerns implantable orthopedic prostheses for total hip replacement and, more particularly, a prosthesis which receives a modular head assembly selected for a desired leg length.
During such surgery, a replacement stem is employed and inserted lengthwise into a pathway in the femur. The stem carries an angularly extending i.e. offset neck trunnion, and a ball at the neck trunnion terminus, to be received into a socket defined by the joint. These components must be accurately relatively positioned to accommodate to each patient's particular femur and hip socket configuration, during surgery, which is time consuming and subject to adjustment problems and difficulties.
When the surgeon implants a definitive femoral prosthesis into the femur, the length and tension of the hip joint is assessed with a loose fitting trial head. The loose fit creates a situation of uncertain evaluation for the surgeon. The stem trunnion moves within the loose fitting trial head much like a piston does in a cylinder, and the surgeon is unable to make an absolute definitive decision as to which head length to implant permanently.
Manufacturers of hip stem prostheses utilize a cold weld (taper fit) of the head and stem trunnion that will unite the stem and the head as one solid unit. In the machining of the stem trunnion, fins are or ribs created that when impacted with the bore of the definitive head, (actual head to be implanted, either metal or ceramic) fold (deform) to create a locking mechanism between the stem and head. This trunnion, including its fins or ribs, are protected from deformity when trialing, by utilizing a loose fitting trial head that won't deform the fins. (the fins are therefore saved from for deforming by the actual head that will be implanted).
When the surgeon fits the femur with a trial stem and a trial head [with or without a trial neck] (some necks are built into the trial stems) the head forms a tight fit to the stem by means of a locking mechanism (a metal c-clip or rubber O-ring) so the surgeon can evaluate:
1) leg length
2) tightness
3) offset
4) dislocation
5) jump distance
6) other relative values
The surgeon decides these relative values of head length with trials that provide an accurate assessment, because the head is captured and restricted from movement upon installation.
After the surgeon has placed (implanted) the definitive hip stem into the patient's femur it is customary to re-trial the head ball because the stem's final seating in the femoral envelope will change from that formed with the previously placed trial stem. The definitive stem can seat itself axially higher, lower, or at the same level as the trial stem, therefore necessitating the need to re-evaluate the head length.
The surgeon then places the same loose fitting trial head onto the definitive stem trunnion and re-evaluates relative values or positions, which are inaccurate because the stem trunnion simply moves axially lengthwise in the head trial. The surgeon wishes to see the head movement in the acetabular liner to evaluate dislocation, jump distance and other relative values necessary for proper head length determination (along the neck) (usual head length variables from −5 to 20 mm in increments et, −5, 2, 0, 1.5, 5, 8.5, 12, 15.5, 18, etc.). This varies by manufacturer.
There is need for apparatus and method to accurately and correctly fit and axially position the trial head ball, relative to it's supporting neck, during surgery.